<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>下拉菜单</title>
    <style>
        .nav>li {
            float: left;
            width: 50px;
            list-style: none;
            height: 100px;
            margin-right: 50px;
            line-height: 41px;
            color: #333;
        }
        .nav>li>a:hover {
            background-color: #eee;
        }
        .nav ul {
            display: none;
            position: absolute;
            top: 41px;
            left: 0;
            width: 100%;
            border-left: 1px solid #fecc58;
            border-right: 1px solid #fecc58;
        }
        .nav ul li {
            border-bottom: 1px solid #fecc58;
        }
        .nav ul li a:hover {
            background-color: #fff5da;
        }
    </style>
</head>
<body>
    <ul class="nav">
        <li>
            <a href="#">微博</a>
            <ul>
                <li>
                    <a href="">私信</a>
                </li>
                <li>
                    <a href="">评论</a>
                </li>
                <li>
                    <a href="">@我</a>
                </li>
            </ul>
        </li>
        <li>
            <a href="#">微博</a>
            <ul>
                <li>
                    <a href="">私信</a>
                </li>
                <li>
                    <a href="">评论</a>
                </li>
                <li>
                    <a href="">@我</a>
                </li>
            </ul>
        </li>
        <li>
            <a href="#">微博</a>
            <ul>
                <li>
                    <a href="">私信</a>
                </li>
                <li>
                    <a href="">评论</a>
                </li>
                <li>
                    <a href="">@我</a>
                </li>
            </ul>
        </li>
        <li>
            <a href="#">微博</a>
            <ul>
                <li>
                    <a href="">私信</a>
                </li>
                <li>
                    <a href="">评论</a>
                </li>
                <li>
                    <a href="">@我</a>
                </li>
            </ul>
        </li>
    </ul>
    <script>
        var nav = document.querySelector('.nav')
        var lis = nav.children //得到4个li
        for(var i = 0; i < lis.length; i++) {
            lis[i].onmouseover = function() {
                this.children[1].style.display = 'block'
            }
            lis[i].onmouseout = function() {
                this.children[1].style.display = 'none'
            }
        }
    </script>
</body>
</html>